A. Field of the Invention
The present invention generally relates to a new and improved magnetic latch for latching the cover of a pressure relief valve in a closed position.
B. Description of the Prior Art
Large volume tanks typically include a pressure or vacuum relief valve to relieve pressure or vacuum that may develop so as to prevent an unsafe condition.
Typically, the pressure or vacuum relief valve includes a valve seat as part of the body and a cover hingedly connected to the body and movable to a position to engage the valve seat in the valve closed position. If an undesirable pressure or vacuum level develops, the cover opens allowing relief of the pressure or vacuum.
One prior art type of the relief valve includes a spring or weight biasing the cover to the closed position. A disadvantage of this prior art valve is that the cover will not undergo a rapid release as undesirable pressures are developed; since the force against which the cover opens with a spring bias increases with the distance the cover has opened, or in the case of a weight biased cover, the force against which the cover opens remains large through the entire opening.
To overcome this disadvantage, several prior art valves provide a magnetic latch that includes a magnet for holding the cover in a closed position. As the cover opens in response to an undesirable pressure or vacuum level in the tank, the force of magnetic attraction between the magnet and the cover rapidly decreases as the distance between the cover and the magnet increases. Consequently, once a slight lifting of the cover occurs, the magnetic bond between the magnet and the cover is sufficiently broken that enough of the biasing force is removed to allow rapid movement of the cover toward the full opened position. Examples of this prior art valve are illustrated in U.S. Pat. Nos. 3,502,361 and 3,528,453.
Several disadvantages result from the employment of the prior art magnetic latches. One disadvantage is that it is necessary to vary the magnitude of the magnetic holding force in accordance with the selection of the pressure level at which the valve is to open. One prior art solution to this disadvantage is to include a plate connected to the cover or base that includes cut out portions that are positioned with respect to the magnet to vary the area of the plate attracted by the magnet and, thus, the force developed between the magnet and the plate. This procedure is somewhat cumbersome to adjust. In addition, it is limited as to the lower limits of adjustment.
A second difficulty with these prior art magnetic latches is that the magnet is exposed to ambient conditions and where ferric particles are present, these particles may short out the poles of the magnet and thus influence the force necessary for opening the valve.